1. Field of the Invention
This invention relates to a process for purifying triphenylmethane compounds which are essentially water insoluble and which contain an aromatic primary amine as an impurity therein. This invention also relates to carbonless copy paper incorporating purified dye precursors, especially those which have been purified by the process of this invention.
2. Description of the Prior Art
Recording sheets of the type which undergo a change in color from a colorless state to a colored state by pressure have long bee known as pressure-sensitive or chemical carbonless copy paper (e.g. as disclosed in U.S. Pat. Nos. 2,711,375; 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; 3,955,025; 4,226,442; etc.). The use of electron donor dye precursors such as Crystal Violet Lactone and Michler's Hydrol paratoluene sulfinate in carbonless copy systems is also known in the art. None of the prior art teaching, however, discloses the use of the purified triphenylmethane compound of this invention as a dye precursor in pressure-sensitive copy paper.
The manufacture of water-insoluble triphenylmethane dyes can be accomplished by methods well known in the art, such as those representatively taught in U.S. Pat. Nos. 3,671,553 and 3,925,094. Therefore, the triphenylmethane dye precursors taught herein can be manufactured, for example, by the following representative methods:
1. By the reaction of rosanilines with primary aromatic amines in the presence of acid catalysts (especially benzoic acid), wherein an amine solution of the color bases is directly obtained (cf. Fierz-David, Kunstl. organische Farbstoffe 1926, page 262 and BIOS Final Report 1433, page 30 et seq.);
2. By the conversion of amine solutions of the color basetetrachloroaluminates, which can be obtained according to German Pat. Nos. 1,098,652, 1,161,370 and 1,161,371 or Belgian Pat. No. 718,410, by reacting 4,4',4"-trichlorotrityl or 4,4'-dichlorotrityl-tetrachlcroaluminates with primary aromatic amines, by means of aqueous alkali metal hydroxide solution into the amine solution of the free color bases. This produces the triphenylmethane compound as a solution in the remaining excess amine;
3. By the reaction of diphenyl amines with aromatic carboxylic acids or their functional derivatives (for example, benzotrihalides, anhydrides or acid halides) in the presence of acid catalysts and subsequent treatment with an aqueous alkali metal hydroxide, whereupon solutions of the color bases are obtained in the aromatic amine used in excess.
Typically, these dyes are utilized as the amine solution since the excess amine acts as a convenient solvent for the triphenylmethane product. There are difficulties inherent in the use of these impure materials, however. First, the intermediates and by-products formed during the manufacture of these compounds, as well as the aromatic amine, remain in the end product and thus reduce its tinctorial strength. Second, the aromatic amine which remains with the dye may constitute a health hazard due to its toxic nature. Third, the yield of products derived from the use of the triarylmethane compounds as reactants (e.g. to produce the monosulfonation products) is reduced due to the presence of the impurities. Fourth, surprisingly, it has now been found that the impurities and reaction by-products derived during the synthesis of these triphenylmethane compounds impart a characteristic which causes "blushing" or premature color development when unpurified dye precursosrs are utilized in carbonless copy paper.
No completely satisfactory method of purification yielding the dye in its carbinol form has been described in the prior art. The triphenylmethane compounds involved in this invention are essentially insoluble in aqueous media in both their carbinol and salt forms. As such, one cannot purify the products to remove the amine and other impurities by slurrying the salt form of the dye in the presence of aqueous caustic and heating to obtain the 100% carbinol form. That obvious route does not go in high yield probably due to the formation of a layer of the insoluble cabinol form of the dye on the surface of the amorphous salt which prevents further reaction. U.S. Pat. No. 3,679,713 teaches a method of purifying water-soluble basic triphenylmethane dyes, but does not specifically teach a process for those triphenylmethane dyes which are water insoluble in both their salt and carbinol form. This patent teaches purification by mixing the water-soluble salt of the dye with a water immiscible solvent and an inorganic base to produce a solvent solution of the color base from which the purified salt can be extracted or precipitated after reacting the color base with an acid.
U.S. Pat. No. 3,652,602 teaches a method of purification which involves separation of the mineral acid salts of the dyes, followed by generation of the carbinol bases by reaction with aqueous alkali hydroxide in an excess of aromatic amine. The aromatic amine can then be distilled off to produce the carbinol base. This process is not totally effective, and it has proven impossible to remove all of the aromatic amine unless such stringent conditions are used which result in partial degradation of the desired carbinol base product.
U.S. Pat. No. 3,671,553 teaches purification of triphenylmethane dyes by treating the amine solution of the dye with excess aqueous sulfuric acid (optionally in the presence of a solvent), filtering the suspension, and distilling off water azeotropically to isolate the color base sulfate.
This invention overcomes the limitations of the prior art by removing the carbinol form of the triphenylmethane dye from the surface of the salt form by subjecting the carbinol form of the dye to temperature higher than about 100.degree. C. under increased pressure so that the carbinol form of the dye can "melt" off the surface of the salt, or alternatively, by generating the carbinol form of the dye in the presence of an organic solvent which is capable of dissolving the dye in its carbinol form.
The triarylmethane or triphenylmethane dyes purified by this invention have special utility when used as reactants for the production of other dyes (e.g. by monosulfonation) or when used as dye precursors in carbonless copy paper.